1. Field of the Invention
The present invention relates to a charging circuit configured to charge a secondary battery using electric power received from a solar battery.
2. Description of the Related Art
In recent years, electronic devices such as cellular phones, PDAs (Personal Digital Assistants), and laptop personal computers, mount a secondary battery. At present, such a secondary battery is charged using electric power received from an adapter configured to convert a commercial AC voltage into a DC voltage, or electric power received from a USB bus. In recent years, there have been efforts to provide an electronic device configured to mount a solar battery, and to charge such a secondary battery using electric power received from such a solar battery.
In a case in which such a solar battery to be used to charge such a secondary battery has a multi-cell structure in which multiple battery cells are stacked, e.g., a structure employing a stack of ten 0.5 V cells, if a part of the solar battery is blocked, the output voltage decreases. In some cases, this leads to a problem in that the charging circuit cannot operate normally. Furthermore, such a solar battery requires wiring space in order to connect the adjacent cells, leading to a problem of a reduced aperture ratio, and also to deterioration in its appearance.
In order to solve such problems, the number of cells for such a solar battery configured to charge such a secondary battery should be on the order of one or two. In a case in which such a solar battery has a single cell, the voltage supplied to the charging circuit is on the order of 0.5 V. In a case in which such a solar battery has two cells, the voltage supplied to the charging circuit is on the order of 1 V. Accordingly, in order to charge a lithium-ion battery (3 to 4.2 V), which is widely employed as a secondary battery, there is a need to boost the voltage received from the solar battery using a DC/DC converter before the voltage is supplied to the secondary battery.
FIG. 1 is a graph which shows the current-voltage (I-V) characteristics of a single-cell solar battery. The horizontal axis represents the voltage output from the solar battery, the left vertical axis represents the output current of the solar battery, and the right vertical axis represents the output electric power of the solar battery. When no load is applied to the solar battery, i.e., when the output current is zero, the output voltage of the solar battery reaches its maximum (open voltage), i.e., 0.6 V. As the output current is greater, the output voltage becomes smaller. Directing attention to the output electric power of the solar battery, the solar battery provides its maximum output electric power when the output voltage is set in the vicinity of 0.5 V. Using such electrical characteristics of the solar battery, a feedback control method has been proposed in which the output voltage of the solar battery, i.e., the input voltage of the DC/DC converter, is stabilized to 0.5 V (the MPP: Maximum Power Point).
However, there is a difference in the maximum power point between a situation in which the illumination intensity is 1000 W/cm2 and a situation in which the illumination intensity is 500 W/cm2. Accordingly, in a case in which feedback control is applied to such a DC/DC converter such that the input voltage approaches 0.5 V regardless of the illumination intensity, such a solar battery does not necessarily provide its maximum power.
Furthermore, the current-voltage characteristics of such a solar battery are endowed with temperature dependence. Specifically, the voltage becomes higher at lower temperatures, and the voltage becomes lower at higher temperatures. Accordingly, with the temperature coefficient as −2 mV/° C., the difference in the maximum power point is 50×2 mV/° C.=100 mV, which is a non-negligible value, between 25° C., which is an ordinary temperature, and 75° C., which is a high temperature. Thus, in order to provide the maximum power point over a possible range of temperatures, there is a need to adjust the input voltage of the DC/DC converter according to the temperature. This leads to a problem of a complicated circuit configuration.
Furthermore, with the MPP method, the electric power supplied to such a secondary battery that acts as a load of the DC/DC converter does not necessarily reach its maximum even if the output voltage of the solar battery, i.e., the input voltage of the DC/DC converter is set to its maximum. This is because electric power is consumed by the DC/DC converter itself.